In known manner such a casing includes a housing for receiving the heat exchanger while leaving the two large faces of its core of heat exchange tubes free for the passage of air. The casing further includes an air inlet duct leading to the upstream large face and an air outlet duct leading away from the downstream large face, together with items such as flaps or the like for controlling the temperature of the air admitted to the cabin by controlling the air flow through the heat exchanger and also for directing the air to various different outlets in the vehicle cabin. The size of the casing, and the shape it has to adopt to fit into the vehicle and connect to sources of heating and/or cooling liquid depends on the space available in the engine compartment and on the locations of various inlet and outlet fittings.
As a result, the air inlet and outlet ducts are often directed towards the corresonding large faces of the heat exchanger at oblique angles which results in a highly non-uniform distribution of air flow through the heat exchanger core. As a general rule the air flows much faster through one end of the core than it does through the other end. Thus, for a given overall flow rate, the heat exchange efficiency of the heat exchanger is generally reduced. This loss of capacity is defined in practice in terms of a matching coefficient for the heat exchanger in the casing, where the matching coefficient is the ratio of the heat exchanged on a test bench (ie. under conditions of uniform air flow) to the heat exchanged by the same heat exchanger when mounted in the casing. The matching coefficient is generally about 0.9 which is equivalent to a loss in exchanger efficiency of 10% due to its being mounted in the casing.
Preferred embodiments of the invention improve the matching coefficient of a given heat exchanger/casing combination by about 5%, without altering the nominal characteristics of the heat exchanger nor the relative orientations of the inlet and outlet ducts relative to the large faces of the heat exchanger.